1. Field of the Invention
The present invention relates generally to IEEE-1394 serial bus networks, and more specifically to speed conversion of packets transmitted between nodes attached to the IEEE-1394 serial buses.
2. Description of the Related Art
IEEE-1394 Serial Bus Standard supports high performance packet transfer at speeds of 100 Mbps, 200 Mbps, and 400 Mbps on a high reliability asynchronous transfer mode (though latency is not guaranteed) and on a bandwidth guaranteed isochronous transfer mode. The IEEE-1394 serial bus is best suited for digital video cameras. With the built-in feature of IEEE-1394 interface, standardized digital video cameras have met with wide reception among consumers.
In a single IEEE-1394 serial bus network, data transfer at different speeds is possible. For example, a 100-Mbps transfer may proceed between two nodes on a cable segment and a 400-Mbps transfer may proceed between other two nodes on a separate cable segment. Since high speed nodes support lower speed transfers, the same node can communicate with a node at a low speed at one time and communicate with another node at a higher speed at different time.
However, from the bandwidth savings viewpoint, the use of a low speed node is not a favorable situation because the time taken to transmit a given amount of information is longer than the time a higher speed node takes to transmit the same amount of information. For example, if a video channel is transmitted on a 100-Mbps isochronous mode, a period of 40 microseconds is required during each 125-microsecond cycle. Since the IEEE-1394 standard specifies that the maximum amount of time available for isochronous transfer for each cycle is 100 microseconds, the maximum number of video channels which the current IEEE-1394 serial bus can support is only two. Therefore, use of different speed nodes in a single IEEE-1394 serial bus network represents a waste of otherwise usable bandwidth resource.